The invention relates to the field of local data networks, and, more particularly, to the field of contention resolution protocols for local data networks using modulated RF carriers.
As the computer using segment of the population has grown, it has become more important to share expensive assets among multiple users. For example in large companies with large mainframe CPU's, their associated peripherals, and large central data bases, as well as numerous local devices, task processors and terminals used by individual employees and subdivisions of the company, it is advantageous to have the remote users be able to share the assets of the mainframe CPU. Thus it is advantageous to have the remote task processors be able to send and receive data from the main CPU, tap the main data bases and be able to print data and store and retrieve data using the printers and magnetic storage media of the main CPU.
Many companies already have existing coaxial cable networks in place to transmit video for security or cable television channels. To avoid having to install a new set of cables for the computer system it is advantageous to be able to use existing video coaxial cable for the distributed data processing system.
Some systems using modulated RF carriers to transmit data exist in the prior art. However, such systems typically utilize repeaters and head end retransmission apparatus and are more expensive. For example, the head end retransmission apparatus of cable television systems is designed to receive data at one frequency and convert it to data of another frequency for retransmission on the cable. The repeaters in the cable have two separate amplifiers, each connected to the same cable, and with one for each direction. Because the output of one amplifier must be connected to the input of the other, unless each works at a different frequency, the repeaters will oscillate. Where different frequencies are used, a head end retransmission unit is necessary. Different cables for transmission and reception could be used but this too requires head end apparatus and the use of two cables is more expensive. It is advantageous to eliminate the need for repeaters and head end apparatus and to use a system and protocol which makes it possible to transmit and receive on the same frequency because a simpler, less expensive system results.
Broadband systems have a major problem with detection of colliding transmissions. In the prior art, there have been schemes proposed to solve this problem. One such scheme adopted by the I.E.E.E. is the so-called token passing scheme. In these systems a token signal is passed around the system and only the unit which has the token can transmit. However, these schemes can be inefficient where the unit which has the token has nothing to transmit at the time and passes the token along but needs to transmit a data packet soon after the token is passed. In such a case, the unit must wait until the token comes around again before it can transmit.
It is important that contention resolution protocols be adopted to avoid colliding transmissions.